1. Field of the Invention
The present invention relates to a thermal printer for image recording on a recording sheet by transferring the ink of an ink sheet to said recording sheet, and is applicable for example in personal computers, word processors, electronic typewriters, facsimiles and so-called printers which record an image on a recording sheet, corresponding to information stored in a memory.
2. Related Background Art
In general, conventional thermal printer has a thermal head containing an array of plural heating elements, and achieves image recording by selectively driving said heating elements while the head is moved, thereby fusing the ink of a thermal transfer ribbon in succession and transferring the ink onto a recording meidum. Such a thermal printer is widely employed as an output device for example in a word processor, as it is compact and light, allows easy maintenance, generates little noise and provides a recording with good preservability.
However, such a thermal printer is associated with a lower recording speed in comparison with wire dot printers or ink jet printers, because, in the thermal printer, the thermal head has to be prevented from overheating for obtaining a satisfactory quality of recording, as will be explained in more detail. For this purpose, in addition to usual heat radiating plate, there is generally provided a cooling period between the driving period of the heating elements. Such cooling period inevitably lowers the recording speed of the thermal printer.
FIG. 1 shows the structure of a conventional thermal head, wherein heating elements 1 are provided on a glaze layer 3 on a substrate 2 and connected at one ends thereof to a common electrode 4 and at the other ends to respective driving electrodes 5.
In addition to the linear array head shown in FIG. 1, there is also known a head having heating elements arranged in a matrix for example 5.times.7. However, such a matrix head is not widely utilized as it requires formation of plural thin layers in contrast to one thin layer formation in the linear array head, and it generally results in a higher cost due to a more complex arrangement of signal lines.
In such a linear array head, the recording is achieved by moving said thermal head in the horizontal direction and applying heating pulses to respective heating elements, but the aforementioned cooling period cannot be made long enough to allow high-speed recording.
In such high-speed recording, the heating pulses are almost continuously supplied to the elements, thus elevating the temperature thereof.
Overheating of the heating elements is undersirable as it not only accelerates deterioration of the elements but also significantly reduces the image quality. Consequently it is difficult to achieve a high recording speed with such a linear array head as the interval of driving is limited by such overheating phenomenon.
In consideration of the foregoing, there has already been proposed a head with two linear arrays of heating elements as shown in FIGS. 2(A) and 2(B). In the structure shown in FIG. 2(A), there are provided two linear arrays A, B of heating elements, symmetrical with the center line of the substrate 2. The elements of the arrays A, B are composed of the same materials as explained before and are represented by the same numbers, with suffixes A, B, in FIG. 2.
The structure shown in FIG. 2(A) has an electrode 4 common to the arrays A and B, and can be prepared with a planar structure substantially the same as that of the linear array head.
On the other hand, FIG. 2(B) shows another thermal head, provided with linear arrays A, B of seven heating elements each. The heating elements are connected at one side thereof to signal electrodes 5A or 5B, and at the other side to a common electrode 4A or 4B. Said heating elements, signal electrodes and common electrodes are provided on the substrate 2, and the heating elements 1A, 1B are provided on a partial glaze layers 3A, 3B formed on the substrate 2.
The thermal head shown in FIG. 2(A) or 2(B), when pressed against a recording sheet 15 on a platen 14 across a thermal transfer ribbon 16 as shown in FIG. 3, can form a doubled number of dots, in comparison with the linear array head, at a head position, if the heads of the arrays A and B are simultaneously driven. Consequently, such a head can achieve a doubled recording speed in comparison with the linear array head.
However, such a thermal head with two linear arrays is associated with a lower print quality in comparison with the thermal head with one array, because of the following considerations. In a recording operation as depicted in FIG. 3, with the head movement in a particular direction in the thermal transfer ribbon 16 is at first heated at the position of the array A, and stores a certain amount of heat when it reaches the position of the array B, positioned downstream in the recording direction.
Consequently the dots obtained with the array B are always denser and larger than those obtained in the array A. Consequently, the quality of printed image is deteriorated due to alternating dot densities between odd and even columns.